Boulder Effects on Bed Load
- The Effects of Hydrodynamic and Granular Controls on Bed Load Flux Intermittency: Application to Steep Mountain Streams (09/01/2012 – 08/31/2014)
- The Role of Relative Submergence on Flow-Obstacle Interaction: Implications to Sediment Transport (04/01/2011 – 03/31/2014)
TeamThanos Papanicolaou, Achilleas Tsakiris
OverviewThese projects focus on the sediment transport processes in mountain stream reaches with arrays of large boulders. Arrays of boulders are naturally found in steep reaches of mountain streams or may be placed artificially in degraded reaches for stream restoration purposes. The additional form drag introduced by the boulder array delays the downstream conveyance of mobile sediment, and introduces significant errors in bedload transport predictions using conventional bedload formulae. The interaction of the boulders with the approaching flow creates a complex field of eddy structures, thus producing a highly spatially variable bed shear stress field in their vicinity, which, in turn, affects the patterns of sediment motion within the boulder array. The objective of the experimental work is to improve our fundamental understanding of the interaction between the boulders and the approaching turbulent flow, which will then be employed for quantifying the effects of the boulder array on the mobile sediment movement.
|Visualization of Flow Around a Boulder||Lab visualization of sand movement over gravel beds|
- Hydraulic and Geomorphic Controls on the Evolution of Cluster Bedforms in Gravel-Bed Streams (08/01/2004 – 07/31/2005)
Thanos Papanicolaou, Achilleas Tsakiris
OverviewThe goal of this research was to investigate the dynamics and morphology of cluster microforms, or simply clusters. Clusters are one type of small-scale bedform in gravel streams, which are broadly viewed as organized of groupings of sediment particles that are the outcome of the complex, two-way interaction between the sediment supply and the turbulent flow in mountainous stream reaches. This research combined field, laboratory as well as numerical/analytical work and revealed that clusters may act as sources or sinks of mobile sediment, and that clusters create a highly complex, 3-dimensional near bed turbulent flow field with implications on the overall resistance to the flow. At the same time, this research identified different cluster morphologies (shapes), which were subsequently verified using fractal analysis, and introduced a statistical model for predicting the occurrence of each cluster morphology using hydraulic, sedimentologic and morphologic parameters at the reach-scale.
Thanos Papanicolaou, Achilleas Tsakiris
OverviewPrevious researchers have documented the formation of fluidization vents, also known as channels or “Monroe volcanoes” in the highly saturated cohesive mud layers found in estuarine tidal flats, or at the bottom of lakes. Large porewater pressure gradients in these mud layers, which are often generated by the settling and self-weight settlement and consolidation of the mud layer or by the wave action, fluidize isolated portions of the mud layer and then water propagates upwards forming these channels. This research project aims to quantify the conditions which signify the onset of channelling in such environments by replicating the channel formation in a controlled laboratory setting using an engineered kaolin mud. Synchronous spatiotemporal observations of the mud layer concentration and the pore-water pressure were acquired via a state-of-the-art fully automated gamma source system and pressure transducers, respectively. Our experiments revealed the existence of two fluidization regimes, namely an advective and a diffusive fluidization regime. Under the advective regime, a single vertical channel formed through the mud layer, whereas under the diffusive fluidization regime multiple channels formed in the mud layer, indicating the formation of a channel network within the mud layer.
|Vent Formation – Advective||Crack Formation – Diffusive|